For devices designed for use in integrated circuits, it is preferred that the main terminals (variously called the anode/cathode, drain/source and emitter/collector) and the control terminals (termed the gate or base) are placed at the surface of the device in order to be easily accessible. The main current flow is between the main terminals and is therefore principally lateral. Such devices are therefore typically referred to as lateral devices. Such devices are often integrated with low-voltage devices or circuits built in CMOS-type or other standard planar technologies to form power integrated circuits. Several high voltage/power devices may be integrated in the same chip. Isolation is provided between the high-power and the low-power devices as well as between adjacent power devices. Two principal isolation technologies have emerged, namely junction-isolation (JI) technology and silicon-on-insulator (SOI) technology.
In JI technology, a reverse-biased junction is used to isolate adjacent devices. However, this is in many cases not satisfactory for power integrated circuits since minority carrier conduction through the semiconductor substrate (on which the active part of the device is formed) can take place and interference between adjacent devices is therefore difficult to prevent. In addition, JI bipolar devices (such as the lateral IGBT) also suffer from parasitic mobile carrier plasma stored in the semiconductor substrate in the on-state which has to be removed during turn-off. This decreases dramatically the switching speed of the devices.
In SOI technology, a buried insulating layer is used to isolate vertically the top semiconductor layer from the bottom semiconductor layer and, accordingly, current conduction is principally restricted to the top semiconductor layer and there is practically no current in the bottom semiconductor layer in any mode of operation. Horizontal or lateral isolation in SOI is typically provided via trenches which are filled with oxide or by use of the known LOCOS (“local oxidation of silicon”) isolation. SOI technology offers better isolation than JI technology because the buried insulating layer prevents current conduction and plasma formation in the substrate.
High voltage semiconductor devices have incorporated within the body of the device a high voltage junction that is responsible for blocking the voltage. This junction includes a relatively lowly doped semiconductor layer which withstands the largest portion of the voltage across the main terminals when the device is in the off-state and operating in the voltage blocking mode. This layer is commonly referred to as the drift region or layer and is partially or fully depleted of minority carriers during this operating mode. Ideally, the potential is equally distributed along the drift region between the two ends of the drift region. However, as shown by the 1-D Poisson equation, for a given doping of the drift region, the distribution of the electric field has a triangular shape or, when fully depleted, a trapezoidal shape. Since the area underneath the electric field can be approximated as the breakdown voltage when the peak of the electric field reaches the critical electric field in the semiconductor, it is obvious that for a 1-D junction, the lower the doping of the drift layer, the higher the breakdown voltage. However, for majority carrier devices such as MOSFET types, known as LDMOSFETs, the on-state resistance of the drift layer is inversely proportional to the doping of the drift layer. Since a low on-resistance is desired for a high voltage switch, it follows that a low doping concentration affects the on-state performance of the device. In addition for lateral devices, the critical electric field at the surface is smaller than in the bulk, adding further difficulties in designing high voltage lateral devices.
The introduction of the RESURF (Reduced Surface Field Effect) technique for JI devices allows an increase in the breakdown voltage of lateral devices through the use of an additional vertical junction formed between the drift region and the semiconductor substrate. FIG. 1a shows schematically a conventional JI diode using the RESURF effect. This diode is provided as part of a conventional lateral power device such as a lateral transistor, LDMOSFET or LIGBT. FIG. 1a also shows the distribution of the potential lines and the edge of the depletion region during the voltage blocking mode. It can be noted that the drift layer 1 is fully depleted but the semiconductor substrate 2 is not fully depleted. The potential lines bend as they drop in the substrate, from the vertical direction towards the horizontal direction, such that below the high voltage terminal 3, the potential lines are practically parallel to the bottom surface 4 of the substrate 2. This is because the thickness of the semiconductor substrate 2 is relatively large (typically 300 μm) compared to the vertical extension of the depletion region from the top surface 5 into the substrate 2 (typically 60 μm for a 600V device). Hence, the semiconductor substrate 2 is not fully depleted when the breakdown of the device occurs. It is known that a lateral JI diode can achieve breakdown voltages equivalent to those of vertical diodes, in spite of the reduced surface critical electric field. Nevertheless, as shown in FIG. 1a, even an optimised electric field distribution using the RESURF concept is far from being ideal (i.e. rectangular in shape). In addition as already mentioned, the JI devices suffer from high leakage currents and very poor isolation, which makes integration within a power integrated circuit very difficult.
FIG. 1b shows a conventional SOI diode which is typically found as part of a SOI lateral high voltage power device. The structure can be made using the known wafer bonding, Unibond or SIMOX SOI technologies. Other technologies such as Silicon-on-Diamond (SOD) are also known. FIG. 1b also shows the equi-potential line distribution during the voltage blocking mode. It can be seen that the potential lines crowd towards the edges of the drift layer 1, resulting in a poor RESURF effect. Increasing the thickness of the buried oxide 6 helps to redistribute the potential lines more evenly at the top surface 5. However, in general, the breakdown voltage is still below that of a JI device or JI diode as shown in FIG. 1a. Again, the potential lines in the drift layer 1 and the buried silicon oxide insulating layer 6 below the high voltage terminal are practically aligned to the horizontal surface. This is due to the fact that the semiconductor substrate 2 is not entirely depleted. The result is that all the potential lines have to. crowd into the drift layer 1 and insulating layer 6 in the case of SOI and moreover have to align parallel to the insulating layer 6/semiconductor substrate 2 interface. This creates an uneven distribution of the potential lines at the top surface 5 which results in high electric field peaks and therefore lower breakdown voltages. In addition, for SOI devices, the conservation of the perpendicular component of the electric flux density D=∈E at the top of the semiconductor layer 1/buried oxide 6 interface limits the maximum voltage that the buried oxide 6 can sustain before the critical electric field in the semiconductor layer 1 at the interface is reached. This vertical breakdown yields a very strong limitation on the maximum voltage rating achievable for a given buried oxide thickness.
Thus, in summary, in both JI and SOI devices, the potential lines have to bend from a vertical orientation to a horizontal or lateral orientation and the potential distribution in the drift layer is far from ideal.
Moreover, when a power integrated circuit made in thin SOI technology comprises at least a half-bridge configuration, which involves two power devices operating in different modes, the device operating in the high side mode may suffer from pinch-off of the drift region during the on-state. This is due to the high electric field in the drift region caused by the high negative potential created in the semiconductor substrate with respect to the potential of one of the main terminals of the high-side device.
It is therefore apparent that the semiconductor substrate in the SOI technology is not passive in all operation modes and its presence results in a poor distribution of the potential lines during the voltage blocking mode, which may cause premature breakdown commonly at the surface of the semiconductor or at the buried oxide/top semiconductor interface due to vertical breakdown. The JI approach suffers from very poor isolation within the power integrated circuit and the breakdown voltage, although generally higher than in the SOI devices, is still lower than would be preferred.
For discrete devices or hybrid circuits used in high voltage or power electronics, it is preferred that the main terminals have a vertical orientation and are placed at opposite sides of the wafer (e.g. with the low voltage terminal at the top and the high voltage terminal at the bottom). These devices are referred to as vertical high voltage/power devices. Compared to lateral devices, the current flow between the main terminals is principally vertical and this results in a larger current capability and a higher breakdown voltage. Such devices are however difficult to use in integrated circuits. Example of known high voltage/power devices are DMOS & Trench MOSFETs, DMOS & Trench IGBTs and Cool MOS.
For an optimised trade-off between on-state/switching/breakdown performance, the vertical devices require a narrow drift region that is preferably fully depleted at full voltage blocking. Such a layer may have a thickness from 6 μm to 180 μm for devices rated from 50 V to 1.2 kV. Commonly the drift layer lies on a highly doped semiconductor substrate. The semiconductor substrate however introduces a series of negative effects on the general performance of the device. First, it introduces a parasitic resistance, which leads to increased on-state power losses. Secondly, for bipolar devices with anode injection such as IGBTs, since the doping of the substrate is high, to reduce the power losses in the substrate resistance, the injection from the substrate which acts as the anode (emitter) of the device is in most cases too strong, leading to high transient switching losses and slow turn-off due to the a large amount of plasma stoned inside the drift region during on-state. Thirdly, the substrate introduces a thermal resistance which prevents effective dissipation of heat to an external sink placed at the bottom of the device. Finally, if vertical devices are to be used in integrated circuits, the presence of the thick semiconductor substrate makes isolation between adjacent devices very difficult.
There have been numerous prior proposals for increasing the breakdown voltage of semiconductor devices, particularly power semiconductor devices. Examples are disclosed in U.S. Pat. Nos. 5,241,210, 5,373,183, 5,378,920, 5,430,316, 5,434,444, 5,463,243, 5,468,982, 5,631,491, 6,040,617, and 6,069,396. However, none of these prior art proposals has tackled the problem of increasing the breakdown voltage by a detailed consideration of the electric potential lines in the drift region.
In WO-A-98/32009, there is disclosed a gas-sensing semiconductor device. A gas-sensitive layer is formed over a MOSFET heater which is used to heat the gas-sensitive layer. The substrate on which the device is formed is back-etched to form a thin membrane in the sensing area. It should be noted that the MOSFET heater is a low voltage device (and as such does not have a drift region) and, furthermore, the thin membrane is formed below the MOSFET heater solely to facilitate heating of the sensing area to very high temperatures and not to affect the field or potential lines in the device.
U.S. Pat. No. 5,895,972 discloses a method and apparatus for cooling a semiconductor device during the testing and debugging phases during development of a device. In place of conventional heat slugs such as copper, a heat slug of material that is transparent to infra red is fixed to the device. A diamond heat slug is disclosed as preferred. It is disclosed that the substrate on which the device is formed can be thinned prior to applying the infra red transparent heat slug to the device. The purpose of this thinning of the substrate is to reduce transmission losses that occur during optical testing and debugging of the device using infra red beams. There is no discussion of the type of semiconductor device to which the heat slug is applied and there is no disclosure that the device is a power device having a drift region. Moreover, as stated, the purpose of the thinning of the substrate and application of the heat slug is solely to facilitate testing of the device using optical testing and debugging. This process is carried out during development of the device. The heat slug is not used during normal operation of the device.
There have been a number of proposals in the prior art for semiconductor devices which make use of a so-called membrane. Examples include U.S. Pat. No. 5,420,458, WO-A-94/22167, U.S. Pat. Nos. 3,689,992 and 6,008,126. In the case of each of these prior art proposals, the semiconductor device is not a power device and thus does not have a drift region. In each case, the membrane arrangement is used to provide for isolation between semiconductor devices in an integrated circuit or between regions within a semiconductor device and/or to remove or lower coupling parasitic capacitances. In each case, since these are low voltage devices, the breakdown voltage is virtually unaffected by the membrane structure.